In Orthogonal Frequency-Division Multiple Access (OFDMA) systems, midamble is a type of measurement pilot that allows a mobile station to obtain channel knowledge between the mobile station and a base station. Midamble transmission is a downlink (DL) signaling mechanism where a base station transmits midamble signals on the downlink to enable the mobile station to estimate base station to mobile station channel response. In one example, the mobile station can use channel knowledge obtained from the received midamble signals to choose the best precoding vector/matrix and then feedback the information back to the base station. In another example, the mobile station can use the channel information to calculate channel quality indicator (CQI) for a specific frequency band. FIG. 1 (Prior Art) illustrates how DL midamble is used for DL close-loop (CL) transmission. In the example of FIG. 1, the base station transmits midamble signals via a midamble channel 11 allocated in DL subframe DL#4 of frame N. The mobile station receives the midamble signals and performs DL channel estimation on the DL channel. In a subsequent frame N+K, the base station transmits data via a data channel 12 in DL subframe DL#2 using cookbook-based DL CL methods such as CL MU-MIMO or CL SU-MIMO.
In IEEE 802.16m systems, a resource block is defined as a two-dimensional radio resource region comprising a number of consecutive sub-carriers (also referred as frequency tones) by a number of consecutive OFDM symbols (also referred as time slots). For both DL and uplink (UL) transmissions, the IEEE 802.16m specification defines various resource blocks such as 5-symbol resource block, 6-symbol resource block, and 7-symbol resource block to be used in various wireless systems. The IEEE 802.16m specification also defines corresponding pilot patterns for various MIMO schemes in each type of resource blocks. FIG. 2 (Prior Art) illustrates examples of different pilot patterns for different MIMO schemes in 6-symbol resource blocks. Resource block 21 is an 18×6 resource block with 8-stream localized pilot pattern, resource block 22 is an 18×6 resource block with 4-stream localized pilot pattern, and resource block 23 is an 18×6 resource block with 2-stream localized/distributed pilot pattern.
To ensure channel estimation quality, midamble signals transmitted via a midamble channel are not allowed to collide with the original pilots that are allocated in various DL resource blocks. Because different base stations may use any of the predefined pilot patterns for data transmission using the same resource block, the midamble channel must not overlap with any of the predefined pilot patterns. As illustrated in FIG. 2, after combining all the predefined pilot patterns, resource regions denoted with letter “P” represent all possible pilot signals transmitted in resource block 14 by different antennas of a base station.
FIGS. 3A to 3C (Prior Art) illustrate examples of different midamble channel patterns allocated for different MIMO schemes. FIG. 3A illustrates midamble channel allocations for 2 Tx and 4 Tx MIMO cases. FIG. 3B illustrates midamble channel allocations for 2 Tx, 4 Tx, and 8 Tx MIMO cases. Similarly, FIG. 3C illustrates different midamble channel allocations for 2 Tx, 4 Tx, and 8 Tx MIMO cases. It can be seen that none of the allocated midamble channels overlap with any of the predefined pilot patterns. Such midamble channel allocation scheme, however, adds undesirable complexity to system implementation. First, each MIMO scheme is associated with a different midamble pattern. This requires base stations and mobile stations to memorize different data mapping rules when applying different MIMO schemes. Second, data and midamble signals co-exist in an OFDM symbol. As a result, it is difficult to control peak-to-average power ratio (PAPR) of the OFDM symbol and thus difficult to determine midamble power boosting.
In addition to the above-described complexity problems associated with midamble channel allocation, other issues arise from midamble sequence design. In existing IEEE 802.16e systems, there are 144 cell IDs defined for base stations located in different cells of an OFDMA system. Each of the 144 cell IDs is assigned to a different midamble sequence for midamble transmission to achieve interference randomization and robust midamble sequence detection for the mobile stations. Each mobile station, however, has to memorize all 144 midamble sequences. To support future network growth (i.e., future femtocell deployment), there are 768 cell IDs defined in current IEEE 802.16m systems. It is no longer feasible to use 768 midamble sequences for each of the 768 cell IDs. A solution is sought.